Keeping clear of kerb curbs chemical contamination

Walking just a few metres away from the edge of city roads makes a real difference to the particulate pollution inhaled by pedestrians pounding the pavement, according to a London research team.

Step away from the kerb: where you choose to walk on a London pavement could affect your health.

The study by scientists from the Centre for Environmental Policy at Imperial College looked at pedestrian exposure to carbon monoxide, ultrafine particles and PM2.5 alongside London's busy Marylebone Road.

During the two-year study volunteers were assigned random routes along the street, and asked to walk on different sides of the street, in different directions and at differing distances from the roadside.

Those whose routes hugged the sides of the buildings that lined the street breathed in far less particulate matter.

Dr Roy Colvile, one of the scientists leading the project, told edie the falloff in exposure to PM2.5, and ultrafine particles in particular, was quite dramatic as you moved away from the kerb.

"If you're in a quiet part of the city these tiny particles don't contribute much to the mass, but as you get closer to busy road they become very significant because of the huge number of them, up to 0.5million per cubic centimetre," he said.

"And those are just the ones our instruments can detect, if you included even smaller ones you're obviously looking at a lot more.

While the results of the study were along the lines predicted by the researchers, the extent of the difference between pollution levels alongside the road and just a few yards away were still surprising.

"We were confident that...the air would be a bit cleaner as you went further from the roads," said Dr Colvile.

"But we weren't sure how much difference it would make.

"It turns out that by walking close to the buildings rather than close to the roads you get a modest but useful benefit in terms of the pollution you are exposed to, something in the region of 20% less."

Dr Colvile said another unexpected discovery was the effect that trees could have when planted alongside roads.

While trees' ability to soak up certain pollutants is well-documented and old news, the study has highlighted the fact that since they force pedestrians away from the roadside simply by being there, they provide an additional unexpected health benefit.

"Nobody had looked at trees like this before," said the scientist.

Interestingly the fixed pollution monitoring station that is a permanent feature of Marylebone Road's streetscape recorded lower levels of both carbon monoxide and PM2.5 than the pedestrian volunteers were exposed to.

"[The monitoring station] is designed to be close to the road to reflect what people might be breathing on the pavement," he said.

"But this thing is static whereas real people move about so they spend time much closer to the traffic, often right on top of it such as when they have to squeeze between the cars to cross the road."

The study, published in the journal Atmospheric Science, adds weight to the case for pedestrians finding alternative routes away from heavy traffic.

"We knew there would be a difference [in exposure] but we were surprised by how much lower concentrations were as you move away from the busy routes," said Dr Colvile.

"Even one block away from these major arterial roads that feed into our cities the air quality is drastically improved."

"These little back streets are beautiful, quiet corridors and it makes sense to use them if you are a pedestrian."

Dr Colvile said Transport for London's recommended cycle routes were a good starting point for navigating the capital without a motor vehicle.

The team's future research will look at other ways the capital's pedestrian population can avoid health risks associated with traffic pollution.

"We want to look in more detail at what people can actually do to reduce their exposure to pollution," said Dr Colvile.

"We also want to look at some specific pollutants, not just what comes out of the cars but some of the stuff that comes off the road surface itself."